Supplemental front wheel drive control system and method

ABSTRACT

An electronic control is provided for controlling the drive speed of a supplemental drive relative to an engine driven main drive of a vehicle. A fluid pump driven by the engine provides pressurized fluid flow. The supplemental drive is driven by the pressurized fluid. A main drive sensor senses the speed of the main drive and responsively produces a main drive speed signal. A command knob is provided for selecting a desired speed relationship between the main and supplemental drives. A knob sensor senses the position of the command knob and responsively produces a modifier signal. A modifier circuit receives the main drive speed and modifier signals and responsively produces a modified main drive speed signal. A supplemental drive sensor senses the speed of the supplemental drive and responsively produces a supplemental drive speed signal. A processor produces an error signal responsive to a difference between received modified main and supplemental drive speed signals. The processor produces a first signal responsive to a product of the error signal and a preselected constant, produces a second signal responsive to an integral of the error signal, produces a third signal responsive to a derivative of the error signal and produces a pump control signal in response to a sum of the first, second, and third signals. An actuator receives the pump control signal and responsively adjusts the pump output.

TECHNICAL FIELD

The present invention relates generally to vehicle drive systems and,more particularly, to an electronic control for a supplementalhydrostatic front wheel drive.

BACKGROUND ART

In motor graders, the rear wheels are commonly driven directly by theengine through a transmission and differential gearing. Further, it iscommon to supplement the main drive by means of a hydrostatic frontwheel drive system. More specifically, the supplemental drive typicallyincludes a fluid pump driven by the engine for providing pressurizedfluid to fluid motors. The motors in turn drive the front wheels therebysupplementing the main rear wheel drive. In the past, supplementalhydrostatic drives have been developed which automatically shiftedbetween two or more torque levels in response to the transmission ratioand/or hydraulic system pressure. These systems were continuouslypowered to provide supplementary hydrostatic drive for the main drivebut had no provision for operation only on demand when the main driveloses traction. However, since the supplemental hydrostatic drive isnotably less efficient than the main direct drive, it is desirable toreduce unnecessary utilization of the hydrostatic drive.

Recent attempts to overcome the problems associated with these systemshave included electronic controls for varying the speed of the frontwheels in response to the speed of the rear wheels. More specifically,these systems commonly utilize speed sensors to monitor front and rearwheel speeds and a closed loop electronic control for varying pumpdisplacement to maintain a preselected speed relationship between thefront and rear wheels.

In some motor graders manufactured by the assignee hereof, the vehicleoperator is able to manually control the torque produced by thesupplemental drive. These vehicles include an open loop control whereinthe pump and motor displacement in the supplemental drive are adjustedin response to a manually operable control lever. However, these systemsdo not provide maximum efficiency since control of the hydrostatic driveis at the operator's discretion. Therefore, it is desirable to provide asystem in which operation of the hydrostatic system is automaticallycontrolled to maintain maximum operating efficiency.

One way in which automatic control of the hydrostatic drive can beachieved is through the use of a closed loop feedback system.

One such system is disclosed in U.S. Pat. No. 4,186,816 which issued toPfundstein on Feb. 5, 1980, hereinafter referred to as '816. The '816patent discloses a closed loop electronic speed feedback system forautomatically controlling the supplemental drive system of a motorgrader. More specifically, closed loop feedback electronics control aservo actuated pump that is connected by a hydraulic system to a pair offront wheel hydrostatic drive motors. The supplemental drive for themotor grader has three modes of operation. The first mode is the "off"mode in which the front drive wheels are free running and unpowered. Thesecond mode is the "normal" mode where the control system allows apredetermined amount speed differential between the main andsupplemental drive wheels before the supplemental hydrostatically drivenwheels begin to supplement the main drive wheels. The third mode is an"overspeed" mode where the control system provides a predeterminedpercentage of overspeed of the auxiliary drive wheels to provide acontinuous, positive supplementary drive.

As mentioned previously, hydrostatic drives are much less efficient thandirect drives and, therefore, it is desirable to utilize thesupplemental drive in the most efficient manner possible. The presentinvention is directed to that end.

DISCLOSURE OF THE INVENTION

An electronic control is provided for a vehicle having an engine, a maindrive driven by the engine and being operative to propel the vehicle. Afluid pump is driven by the engine and is responsive to providepressurized fluid. A supplemental drive is driven by the pressurizedfluid and is operative to propel the vehicle. The control includes maindrive sensor for sensing the speed of the main drive and responsivelyproducing a main drive speed signal. A command knob is provided forselecting a desired speed relationship between the main and supplementaldrives. A knob sensor means senses the position of the command knob andresponsively produces a modifier signal. A modifier circuit receives themain drive speed and modifier signals and responsively produces amodified main drive speed signal. A supplemental drive sensor isprovided for sensing the speed of the supplemental drive andresponsively producing a supplemental drive speed signal. A processorreceives the modified main and supplemental drive speed signals andproduces an error signal responsive to a difference between the receivedsignals. The processor produces a first signal responsive to a productof the error signal and a preselected constant, produces a second signalresponsive to an integral of the error signal, produces a third signalresponsive to a derivative of the error signal and produces a pumpcontrol signal in response to a sum of the first, second, and thirdsignals. An actuator receives the pump control signal and responsivelyadjusts the pump so as to vary the pressure of the pressurized fluidproduced by the pump in response to the pump control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an industrial motor grader schematicallyillustrating the general location of most of the principal drivecomponents including the supplemental hydrostatic drive;

FIG. 2 is a schematic illustration of an electronic control of oneembodiment of the present invention;

FIGS. 3A-B are schematic illustration showing generally the constructionof the hydraulic motors;

FIGS. 4A and 4B disclose a block diagram of the electronic control ofFIG. 2; and

FIGS. 5A-E are flow diagrams of certain functions performed by anembodiment of the immediate vehicle drive control.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, there is shown a work vehicle 10 and, moreparticularly, an industrial motor grader 11 having a main internalcombustion engine 12 driving a main drive 14. The main drive 14 includesa pair of rear wheels 16 driven by the engine 12 through a conventionalelectronically controlled and hydraulically actuated transmission 18 anda rear differential 19, as is common in the art. The transmission 18 isresponsive to a gear selector 20 and a clutch pedal 22 which are bothlocated in an operator's compartment 24 of the motor grader 11.

More particularly, a clutch pedal sensor 26 is provided for sensing theposition of the clutch pedal and responsively producing a clutch pedalposition signal. Preferably, the clutch pedal sensor 26 is in the formof an electrical switch (not shown) which is connected to ground whenthe clutch pedal 22 is depressed and open potential when the clutchpedal 22 is released. Similarly, a gear selector sensor means 28produces a gear selector signal responsive to the position of the gearselector 20. The gear selector 20 is movable between eight forward gearpositions, a neutral position and six reverse gear positions. The gearselector sensor produces a unique output for each of these positions.The clutch pedal and gear selector position signals are delivered totransmission solenoids (not shown) for controlling actuation of thetransmission 18 in a conventional manner.

Referring additionally now to FIG. 2, a supplemental hydrostatic frontwheel drive 30 for the motor grader 11 will now be discussed. Thesupplemental drive 30 includes left and right front drive wheels 32a,32bdriven by respective hydraulic motors 34a,34b. A hydraulic pump 36 isdriven by the engine 12 for providing pressurized fluid to the motors34a,34b. The hydraulic pump 36 is a reversible variable displacementpump as is common in the art. An actuator means 38 receives a controlsignal from an electronic control 31 and responsively adjusts thedirection and displacement of the pump 36. More specifically, theactuator means 38 includes forward and reverse pump actuators (notshown) which are responsive, respectively, to forward and reverse pumpcontrol signals produced by the electronic control 31. The electroniccontrol 31 selectively delivers either the forward or reverse pumpcontrol signal to the actuator means 38 over first and second electricalconductors 39a,39b, respectively, in response to the direction of themain drive 14. It should be understood that a nonreversible pump couldbe utilized in conjunction with a reversing valve for supplying thepressurized fluid to the motors 34a,34b, as opposed to using thereversible variable displacement pump 36.

The pressurized fluid is supplied to the motors 34a,34b through ahydraulic system 40 to power the motors 34a,34b and drive the wheels32a,32b. The hydraulic motors 34a,34b are rotating housing, radialpiston designs, one of which is shown generally in FIGS. 3A-B. It shouldbe understood that the type of hydraulic motor used forms no part of theimmediate invention and, therefore, only a brief description of themotors 34a,34b will be given. Each motor 34a,34b includes a plurality ofpistons 42 having respective rollers 43 which work outwardly against acam ring 44 to impart rotary motion in the cam ring 44. The cam ring 44are rigidly connected to and rotatable with respective drive wheels32a,32b for propelling the vehicle. Each motor 34a,34b includes arotating distribution valve (not shown) which is timed to selectivelydirect fluid to and from the pistons 42. In the preferred embodiment,the hydraulic motors 34a,34b are a model H20 as manufactured by Poclain,Inc.

The motors 34a,34b operate at either 100% or 40% displacement. Theelectronic control 31 determines the gear ratio of the main transmission18 and responsively delivers a motor displacement control signal to amotor displacement control means 46 for controlling the displacement ofthe motors 34a,34b. The motors 34a,34b operate at 100% displacement inforward gears 1-4 and reverse gears 1-3, and 40% displacement in forwardgears 5-7 and reverse gears 4-5. The motor displacement control means 46includes an on/off solenoid actuated valve (not shown) for controllingfluid flow to the pistons 42 in response to the motor displacementcontrol signal. More specifically, in the 100% mode the solenoidactuated valve is deengergized, thereby causing the distribution valveto selectively direct fluid flow to all of the pistons 42a-j. Whereas,in the 40% mode, the solenoid actuated valve is energized therebycausing the distribution valve to selectively direct fluid flow to only40% of the pistons 42. Since the pressurized fluid is being distributedbetween fewer pistons 42 in the 40% mode, the pistons 42 act morerapidly, thereby causing the cam rings 44 to rotate at a higher speedfor a given fluid pressure.

The hydraulic system 40 also includes a solenoid actuated freewheelvalve 48, hereinafter referred to as the freewheel valve 48, which isresponsive to a freewheel control signal from the electronic control 31for controlling fluid flow to the motors 34a,34b. More specifically,when the freewheel solenoid 48 is deenergized, the pistons 42 in eachmotor 34a,34b are fully retracted as shown in FIG. 4b. When this occursthe contact between the rollers 43 and the cam rings 44 is broken whichallows the associated wheels 32a,32b to rotate freely. The freewheelmode is obtained by pressurizing the motor case (not shown) through adrain port (not shown) and simultaneously connecting the distributionvalve to tank (not shown) with zero back pressure. In the absence offluid pressure, the pistons 42 are biased to their retracted position byrespective bias springs (not shown). Production of the freewheel controlsignal will be explained in greater detail below in connection with theelectronic control 31.

Referring now again to FIG. 2, the electronic control 31 will beexplained in greater detail. The electronic control 31 includes acontroller means 50 which is electrically connected to a variety ofsensors through conventional conditioning circuits (not shown) forreceiving respective input signals. The controller means 50 processesthese signals and responsively delivers a plurality of control signalsto the supplemental drive 30 to effect operation of the supplementaldrive 30 in a manner as explained below. The controller means 50 may beimplemented with any suitable hardware including analog or digitalcircuits which may be either discrete components or integrated circuits.However, in the preferred embodiment, the controller means 50 isimplemented employing a microprocessor 52 having external RAM and ROM(not shown) and being programmed to control operation of thesupplemental drive 30 as explained below. A number of commerciallyavailable devices are adequate to perform the control functions, such asthe MC6800 series components manufactured by Motorola, Inc. Using amicroprocessor in such a system is preferable because the overall systemis simplified in comparison to a system embodied in discrete componentsor integrated circuits. Furthermore, a microprocessor based system canbe modified easily through software changes and updates.

A main drive sensor means 54 senses the speed of the main drive 14 andresponsively produces a main drive speed signal. The main drive sensormeans 54 includes an engine speed sensor means 56 for sensing the speedof the engine 12 and producing an engine speed signal. The engine speedsensor means 56 can be any type of sensor that accurately produces anelectrical signal in response engine crankshaft speed. However, in thepreferred embodiment, the engine speed sensor means 56 includes amagnetic pick-up sensor 58 mounted on an engine flywheel housing (notshown) for sensing rotation of a toothed gear 60 which rotates at aspeed responsive to engine crankshaft speed. The magnetic pick-up sensor58 produces an electrical signal having a frequency responsive to therotational speed of the gear 60 and thus engine speed.

The main drive sensor means 54 further includes a transmission sensingmeans 62 for sensing the transmission's gear ratio and responsivelyproducing a gear ratio signal. More specifically, the controller means50 is electronically connected to the transmission solenoids (not shown)for detecting the gear position of the transmission 18. An energizedsolenoid is associated with logic "0" and a deenergized solenoid isequated to logic "1". For each transmission gear ratio, a unique gearcode signal is produced in response to the energization state of thetransmission solenoids. The controller means 50 receives the gear codesignal and accesses a lookup table stored in memory to determine thetransmission gear ratio and responsively produces a gear ratio signal.The controller means 50 further receives the engine speed signal,calculates the speed of the main drive speed in response to the enginespeed signal and gear ratio signals, and produces a main drive speedsignal corresponding to the calculated speed. The main drive sensormeans 54 could take numerous other forms without departing from thescope of the invention. For example, it could be embodied in a speedsensor operatively associated with an output shaft (not shown) of thetransmission 18 for sensing the rotational speed of the shaft andresponsively producing a signal corresponding to the speed of the maindrive 14.

A supplemental drive speed sensor means 64 is provided for sensing thespeed of the supplemental drive 30 and responsively producing asupplemental drive speed signal. The supplemental drive speed sensormeans 64 includes first and second speed transducers 66a,b forrespectively sensing the speeds of the left and right drive wheels 32a,32b and responsively producing respective first and second speedsignals. The first and second speed transducers 66a,b produce electricalsignals having frequencies responsive to the speed of the left and rightfront drive wheels 32a,32b, respectively. The processor means 50receives the first and second speed signals and produces thesupplemental drive speed signal in response to the average of the firstand second signals. The average front wheel speed is used to providemore accurate indication of the front wheel speed than could be obtainedby sensing the speed of only one wheel 32a,32b. For example, whenvehicle is cornering, the outside wheel rotates faster than does theinside wheel. Therefore, sensing the speed of one of the wheel gives aninaccurate indication of the true front wheel speed.

A mode switch 68 is provided for producing a mode signal correspondingto a desired operating mode for the supplemental drive 30. The modeswitch 68 is in the form of a two position switch where the positions,indicated by "O" and "A" correspond respectively to "off" and"automatic" modes of supplemental drive operation. In the "off" mode,the front drive wheels 32a,32b are free running and unpowered. While inthe "automatic" mode, the fluid pressure in the supplemental drive 30 iscontrolled to maintain a selected speed relationship between the mainand supplemental drives 14,30.

A command knob 70 is provided to allow the vehicle operator to selectthe speed relationship to be maintained between the main andsupplemental drives 14, 30 during the "automatic" mode. In the preferredembodiment, the supplemental drive 30 can be operated between 0 and 10%faster than the main drive 14. The command knob is movable between adiscrete number of positions to enable the operator to select the speedrelationship between the main and supplemental drive 14,30. A commandknob sensor means 72 senses the position of the command lever 70 andproduces a modifier signal in response the position of the command lever70. The modifier signal is in the form of a pulse width modulated (PWM)signal having a duty cycle corresponding the position of the commandknob 70.

A pressure sensor means 74 senses the pressure of the pressurized fluidsupplied to the front wheel motors 34a,34b by the pump 36 andresponsively produces an actual pressure signal. The pressure sensormeans 74 is in the form of a pressure transducer 76 which produces anelectrical signal having a frequency responsive to the output pressureof the pump 36.

The controller means 50 is electrically connected to the clutch pedalsensor means 26, the transmission sensing means 62, the speedtransducers 66a,66b, the mode switch 68, the command knob sensor means72 and the pressure transducer 76 for respectively receiving the clutchpedal position, gear code, first and second speed, mode, modifier, andactual pressure signals. The controller means 50 processes these signalsto control the supplemental drive 30 in a manner set forth below.

Referring now to FIGS. 4A and 4B operation of the controller means 50will be discussed in greater detail. In FIG. 4, the electrical signalshave been assigned references to more readily facilitate the descriptionof the drawing. The controller means 50 includes a calculator means 78for receiving the gear code and engine speed signals (GC, ES),determining the speed of the rear wheels, and responsively producing amain drive speed signal (MDS). A modifier means 80 receives the maindrive speed and modifier signals (MDS, MOD) and responsively produces amodified main drive speed signal (MMDS). More specifically, the modifiedmain drive speed signal (MMDS) is produced in accordance with thefollowing formula:

    MMDS=MDS*(1+MOD)

As can be seen from the above equation, the modified main drive speedsignal (MMDS) will always be greater than or equal to the main drivespeed signal (MDS).

The controller means 50 further includes an averaging means 82 whichreceives the first and second speed signal (FSS, SSS) and responsivelyproduces the supplemental drive speed signal (SDS). As mentionedpreviously, the supplemental drive speed signal (SDS) is indicative ofthe average of the first and second speed signals (FS, SS) and thus theaverage speed of the front drive wheels 32a,32b. The modified main(MMDS) and supplemental drive (SDS) speed signals are delivered to afirst summing means 84 which produces a error signal (e_(S)) in responseto a difference between the received signals.

The error signal (e_(S)) is delivered to a PID means 86 which processthe error signals to produce a pump control signal (PC). The PID means86 includes a proportion calculating means 88 which produces a firstsignal (FS) responsive to a product of the error signal (e_(S)) and apreselected constant. An integral calculating means 90 receives theerror signal (e_(S)) and produces a second signal (SS) responsive to anintegral of the error signal (e_(S)). A derivative calculating means 92receives the error signal and produces a third signal (TS) responsive toa derivative of the error signal e_(S). A second summing means 94receives the first, second, and third signals (FS, SS, TS) and producesthe pump control signal (PC) in response to a sum of the receivedsignals. More particularly, the PID means 86 produces the pump controlsignal (PC) at least in part according to the following formula:

    PC=K.sub.1 e.sub.P +K.sub.2 Σe.sub.P +K.sub.3 Δe.sub.P

where

K₁, K₂, K₃ are predetermined constants.

In an application such as the present supplemental drive 30, a PIDcontrol is advantageous over a proportional control or a proportionalintegral control because it is more responsive and has better stabilityin the absence of the error signal (e_(S)). More specifically, thesecond term, or the integrating term, is provided to keep the pump 36 atthe proper displacement in the absence of the error signal (e_(S)). Ifthe integrating term is not used, some type of servo mechanism, such asa mechanical servo, must be provided to accomplish this function. Thisis undesirable because it adds to the cost and complexity of thesupplemental drive system 30. The third term, or the differentiatingterm, makes the control more responsive because it responds to the rateof change of the error signal (e_(S)). As such it serves to predict thefuture behavior of the error signal (e_(S)), thereby allowing thecontrol to respond more quickly to changes in the error signal (e_(S)).

A pump signal generator means 96 receives the mode, clutch pedalposition, gear code, actual pressure, and pump control signals (MS, CP,GC, AP, PC). The pump signal generator means 96 processes these signalsto determine the current operating mode of the main drive 14 andcontrols the supplemental drive 30 accordingly. More specifically, thepump signal generator means 96 sets the magnitude of the forward andreverse pump current signals (FPC, RPC) in response to the receivedsignals. The forward and reverse pump current signals (FPC, RPC) aredelivered to the forward and reverse pump actuators via the conductors39a,39b, respectively, thereby controlling pump direction anddisplacement in a conventional manner.

First the pump signal generator examines the mode signal (MS) todetermine if the operator has requested the "off" or "automatic" mode.If the signal indicates the "off" mode, the forward and reverse pumpcurrents signals (FPC, RPC) are both set at zero. The present system isadvantageous because the system electronics remain active during the"off" mode. More specifically, the controller means 50 produces the pumpcontrol signal (PC) during both the "off" and "automatic" modes.However, during the "off" mode delivery of the control signal to toactuator means 38 is suppressed. Therefore, if an operator subsequentlyrequests the "automatic" mode, the control responds quickly to thisrequest.

If the mode signal (MS) indicates the "automatic" mode, the pump signalgenerator 96 first checks the actual pressure signal (AP) to determinethe fluid pressure in the supplemental drive is above or below first andsecond preselected limits (L1,L2), respectively. The first preselectedlimit (L1) is set at the maximum allowable system pressure of 5000 psias determined by design criteria. Operation at pressures higher than thesystem pressure are extremely inefficient and can result in fluid lossesdue to leakage. If the actual pressure signal indicates that thepressure is above 5000 psi, the pump signal generator 96 decreases thepump control signal (PC) by a first preselected amount (I1). The pumpcontrol signal (PC) is repeatedly decremented by the first preselectedamount (I1) until the actual pressure signal (AP) indicates that thepump pressure is below 5000 psi.

The second predetermined limit (L2), or lower pressure limit, is set toensure that the piston rollers 43 and cam rings 44 remain engaged duringoperation of the supplemental drive 30. If the pressure is allowed todrop below the second preselected limit, it is possible for the motors34a,34b to be damaged. More specifically, at pressures lower than thesecond preselected limit (L2), the pistons 42 only partially extend. Asthe cams rings rotate 44, due to movement of the vehicle 10, the camrings 44 collide with the partially extended pistons 42, therebydamaging the motors 36a,b. The lower limit is dependent on thespecifications of the motors 34a,34b and the hydraulic system 40, andhas been empirically determined to be 700 psi for the immediatesupplemental drive 30. If the actual pressure signal (AP) indicates thathydraulic pressure is below 700 psi, the pump signal generator 96increments the pump control signal (PC) by a second preselected amount(I2). The pump controller means 96 repeatedly increments the pumpcontrol signal by the second preselected amount (I2) until the actualpressure signal (AP) indicates that the pump pressure exceeds 700 psi.

The pump signal generator 96 then examines the gear code signal (GC) todetermine the operating mode of the main transmission 18. If the gearcode signal (GC) indicates that the main transmission 18 is in 8th gearforward, neutral, 6th gear reverse the forward and reverse pump currentsignals (FPC, RPC) are both set to zero. The forward and reverse pumpcurrent signals (FPC, RPC) are also set to zero if the clutch pedalposition signal (CP) indicates that the clutch pedal 22 is depressed. Ifthe transmission 18 is in forward gears 1-7 the forward pump currentsignal (FPC) is set to the value of the pump control signal (PC) and thereverse pump current signal (RPC) is set to zero. Conversely if thetransmission 18 is in reverse gears 1-5 the reverse pump current signal(RPC) is set to the value of the pump control signal (PC) and theforward pump current signal (FPC) is set to zero.

A motor displacement selector means 100 receives the gear code signal(GC) and responsively produces the motor displacement signal (MD). Themotor displacement signal (MD) is in the form of a current signal whichis either "off" or "on" at a level required to energize the solenoidactuated valve of the motor displacement means 46. When the solenoidactuated valve is energize, the motors are at 40% displacement and whenthe solenoid actuated valve is deenergized the motors are at 100%displacement. The motor displacement selector 100 checks the gear codesignal (GC) and sets the magnitude of the motor displacement signal (MD)appropriately. More specifically, if the gear code signal (GC) indicatesthat the transmission 18 is in forward gears 1-4 or reverse gears 1-3,the motor displacement signal (MD) is "off", thereby deenergizing thesolenoid actuated valve. When the transmission is in forward gears 5-8or reverse gears 4-6, the motor displacement signal (MD) is "on",thereby energizing the solenoid actuated valve.

A freewheel driver means 102 receives the mode, gear code, and clutchposition signals (MS, GC, CP) and selectively produces a freewheelsignal (FW) which is delivered to the freewheel solenoid 48. Thefreewheel signal (FW) is either "off" or "on" at a magnitude required toenergize the freewheel solenoid 48. The freewheel signal (FW) is "off"if the mode selector 68 is in the "off" position or if the clutch pedal24 is depressed. Furthermore, the freewheel signal (FW) is "off" if themain transmission 18 is in neutral, in forward gear 8 or reverse gear 6,thereby preventing operation of the supplemental drive 30 at high groundspeeds.

Referring now to FIGS. 5a-e, flow diagrams which can be used to programthe microprocessor 52 to perform certain functions immediate vehicledrive control will be discussed. Initially, in the block 200, the inputsignals from the various sensor are read and variables in memory are setin accordance with the sensed values. Next in the block 210, the speedof the main drive is determined in response to the gear code and enginespeed signals (GC, ES). More specifically, the gear code signal (GC) isused to access a lookup table stored in memory for determining the gearratio of the main transmission 18. The lookup table produces a uniquegear ratio signal (GR) for each transmission gear ratio in response tothe gear code signal (GC). The gear ratio and engine speed signals (GR,ES) are used in the following formula, which is stored in memory andaccessed to produce the main drive speed signal: ##EQU1##

where R represents the rolling radius of the main drive tires.

Thereafter, in the block 215, the supplemental drive speed signal (SDS)is produced response to an average of the first and second speed signals(FSS, SSS). Control is then passed to the block 220 where the modifiedmain drive speed signal (MMDS) is produced in response to the modifierand main drive speed signals (MOD, MDS). More specifically, the modifiedmain drive speed signal (MMDS) is produced in accordance with thefollowing formula:

    MMDS=MDS*(1+MOD)

Next, in the block 225, the error signal (e_(S)) is calculated inresponse to a difference between the modified main and supplementaldrive speed signals (MMDS, SDS). Control is then passed to the block 230where the pump control signal (PC) is calculated in accordance with thefollowing equation:

    PC=K.sub.1 e.sub.P +K.sub.2 Σe.sub.P +K.sub.3 Δe.sub.P

where K₁, K₂, K₃ are predetermined constants stored in memory.

Control is then passed to a pump signal generator routine which is shownin the blocks 240-330. The pump signal generator routine processes themode, clutch pedal position, gear code and pump control signals (MS, CP,GC, PC) to control operation of the supplemental drive 30. Initially, inthe block 240, the mode signal (MS) is examined and control is passed tothe block 245 if the mode signal (MS) does not corresponds to the"automatic" mode of operation. In the block 295 the forward and reversepump current signals (FPC, RPC) are both set to zero and then control ispassed to the block 335 to begin execution of a freewheel routine asexplained below.

Otherwise control is passed to the block 250 where the actual pressuresignal (AP) is examined. If the actual pressure signal (AP) indicatesthat the pump pressure is above 5000 psi, the pump signal generator 96decreases the pump control signal (PC) by a first preselected amount(I1) in the block 260. However, if the actual pressure signal (AP) isless than the first predetermined limit (L1), control is passed to theblock 265. In the block 265 the actual pressure signal is compared to asecond predetermined limit (L2), and control is passed to the block 270if the actual pressure signal (AP) is less than or equal to the secondpredetermined limit (L2). In the block 270 the pump control signal (PC)is incremented by a second preselected amount (I2).

Control is then passed to the block 300 where the gear code and clutchpedal signals (GC, CP) are examined. If the clutch pedal 22 is depressedor the transmission 18 is in neutral, control is passed to the block 305where the forward and reverse pump current signals (FPC, RPC) are bothset to zero and then control is routed to the block 335.

If the tests in block 300 are negative, control is passed to the block310 where the gear code signal (GC) is examined to determine if thetransmission 18 is in a forward gear. If it is, control is passed to theblock 315 where forward pump current signal (FPC) is set to the pumpcontrol signal (PC) and reverse pump current signal (RPC) is set tozero.

Otherwise, control is passed to the block 320 where the gear code (GC)signal is again examined to determine if the transmission 18 is in areverse gear. If it is, control is passed to the block 325 where reversepump current signal (RPC) is set to the pump control signal (PC) andforward pump current signal (FPC) is set to zero.

Conversely, if in the block 320 it is determined that the transmission18 is not in a reverse gear, the gear code signal (GC) is assumed to beinvalid and control is passed to the block 330. In the block 330 theforward and reverse pump current signals (FPC, RPC) are both set tozero.

Thereafter, control is passed to the block 335. The block 335 is thestart of a freewheel control routine which continues through the block370. In the block 335 the mode signal (MS) is examined and control ispassed to the block 345 if the mode signal (MS) does not correspond tothe "automatic" mode. In the block 345 the freewheel signal (FW) isturned "off", thereby deenergizing the freewheel solenoid 48 and causingthe motors 34a,b to "freewheel" as explained above. From the block 345,control is passed to the block 375.

Otherwise control is passed to the block 350 where the clutch pedalsignal (CP) is examined to determine if the clutch pedal is depressed.If it is, control is passed to the block 355 where the freewheel signal(FW) is turned "off". This is done to disable the supplemental drive 30when the main transmission 18 is in neutral. Control is routed to theblock 375 from the block 355.

However, if the clutch pedal is not depressed, control is passed to theblock 360 where the gear code signal (GC) is examined to determine theoperating gear of the main transmission 18. If the transmission 18 is inneutral, forward gear 8, or reverse gear 6 control is passed to theblock 365 where the freewheel signal (FW) is turned "off" and control isthen passed to the block 375. Otherwise, control is passed to the block370 where the freewheel signal is turned "on" causing the freewheelsolenoid 48 to become energized.

In the block 375 the gear code signal (GC) is again examined todetermine the operating mode of the main transmission 18. If the gearcode signal (GC) indicates that the main transmission 18 is in forwardgears 5-8 or reverse gears 4-6, the motor displacement signal (MD) isturned "on", thereby causing the motors to operate at 40% displacement.Otherwise, the motor displacement signal (MD) is turned "off", therebycausing the motors to operate at 100% displacement.

The above supplemental drive routine is repeatedly executed throughoutoperation of the vehicle 10 for producing efficient operation of thesupplemental drive 30. Other aspects, objects, and advantages of thisinvention can be obtained from a study of the drawings, the disclosure,and the appended claims.

Industrial Applicability

Assume that the supplemental drive 30 is initially in the "off" mode.The processor means 52 means produces the error signal (e_(S)) inresponse to a difference between supplemental and modified main drivespeed signals (SDS, MMDS). The error signal (e_(S)) is delivered to thePID means 86 which produces the pump control signal (PC) in response tothe error signal. However, the pump signal generator means 96 sets boththe forward and reverse pump currents (FPC, RPC) to zero in response tothe mode signal (MS) indicating the automatic mode. Furthermore, thefreewheel driver means 102 receives the mode signal (MS) and sets thefreewheel signal to "off" in response to the mode signal indicating the"off" mode. This causes the motor pistons 42 to disengage the cam rings44, thereby allowing the front wheels rotate freely in response tomovement of the vehicle.

At some point in time, the operator moves the mode switch 68 to the"automatic" mode. The pump signal generator means 96 senses this andthen processes the gear code (GC) signal to determine the operating modeof the main transmission 18. If the main transmission is in 8th gearforward, neutral, or 6th gear reverse, the forward and reverse pumpcurrent signals (FPC, RPC) are both set to zero. However, if none ofthese conditions are met, the pump signal generator means 96 deliverseither the forward or reverse pump current signals (FPC, RPC) to theforward and reverse actuators, respectively, in response to thedirection of the main transmission 18. This causes the pump 36 tooperate in a direction and at a displacement which tends to minimize theerror signal, thereby maintaining the desired speed relationship betweenthe main and supplemental drive 14, 30. Furthermore, the freewheeldriver means 102 senses that the mode signal (MS) indicates the"automatic" mode and responsively turns the freewheel signal (FW) "on".This causes the pistons 42 to engage and drive the cam rings 44, therebydriving the front wheels 32a ,32b at the desired speed.

We claim:
 1. A work vehicle (10) having a main drive, a fluid pump (36)for providing pressurized fluid, and a supplemental drive (30) driven bysaid pressurized fluid, said main and supplemental drives each beingrotatable at a desired speed, comprising:main drive sensor means (54)for sensing the speed of the main drive (14) and responsively producinga main drive signal; supplemental drive sensor means (64) for sensingthe speed of said supplemental drive (13) and responsively producing asupplemental drive speed signal; a mode switch means for selectivelyproducing "off" and "Automatic" mode signals; a microprocessor (52) forreceiving said mode, main drive speed and supplemental drive speedsignals, modifying said main drive signal by a preselected amount andresponsively producing a modified main drive speed signal, producing anerror signal responsive to a difference between said supplemental andmodified main drive speed signals, and processing said error signal toproduce a control signal, said microprocessor delivering said controlsignal in response to said mode signal being "Automatic" and inhibitingthe delivery of said control signal in response said mode signal being"off"; and actuator means (38) for receiving said control signal andresponsively adjusting said pump (36) so as to vary the rate of flow ofpressurized produced by said pump (36) in response to said controlsignal.
 2. A method of controlling a supplemental drive (30) in avehicle (10) having a main drive (14) for propelling said vehicle, and afluid pump (36) for delivering fluid to said supplemental drive (30) todrive the same and provide supplemental propulsion for said vehicle(10), said main and supplemental drives each being rotatable at aselected speed, comprising the steps of:selecting one of an "off" and"Automatic" mode of operation; sensing the speed of the main drive (14)and responsively producing a main drive speed signal; sensing the speedof said supplemental drive (30) and responsively producing asupplemental drive speed signal; producing an error signal responsive toa difference between said received signals; producing a first signalresponsive to a product of said error signal and a preselected constant;producing a second signal responsive to and integral of said errorsignal; producing a third signal responsive to a derivative of saiderror signal; producing a control signal responsive to a sum of saidfirst, second, and third signals irrespective of the mode of operationselected; blocking said control signal in response to said "off" mode ofoperation being selected and passing said control signal in response tosaid "Automatic " mode of operation being selected; controlling the flowof fluid in said supplemental drive (30) in response to the controlsignal being passed.
 3. A method as set forth in claim 2 wherein: thepump (36) is a variable displacement pump; the step of producing acontrol signal includes delivering the control signal to the variabledisplacement pump; and the step of controlling the flow of fluid in saidsupplemental drive includes varying the displacement of the pump tocontrol the flow of fluid to the supplemental drive (30).